Response-Based, Feedforward Wide-Area Control

I suggest R&D directions for wide-area angle and voltage stability controls in large power systems. Existing wide-area stability controls are mostly event-driven discontinuous actions, and are termed special protection systems or remedial action schemes. More sophisticated controls act on the measured response (swing) of electrical variables to disturbances. I describe applications of response-based, feedforward (disturbance-rejecting) wide-area controls, including preliminary designs for stability and voltage controls in the Pacific Northwest portion of the western North American interconnected power system. Power System Stability Control Challenges Industry restructuring poses challenges in facilitating commerce over transmission networks while ensuring power system security against cascading outages [1]. There are many areas requiring R&D and technology deployment. Experience indicates technology deployment will likely lead to new areas for research. Because of the difficulties and lack of incentives to expand transmission networks, transmission line loadings are increasing as load grows and commerce increases. The power system stress and potential for synchronous and voltage instability are increasing. There is pressure to relax reliability criteria, even to the point of operating to N-0 criterion. This means reliable and robust control actions will be required for many single contingencies. These control actions are often of the discontinuous, feedforward (disturbance-rejecting) type. (Spinning reserve requirements and requirements for predisturbance load curtailment are already being replaced with load tripping in the event generation or import capability is actually lost.) The high volatility in power prices challenges the industry to develop IT solutions for direct load control. Direct load control (air conditioners, space and water heating) also offers a painless way to shed load for power system stability, especially longer-term voltage stability. With the digital/optical revolution in sensors, control and protection, and communications, system reliability can be greatly enhanced by replacement of legacy equipment. The challenge is to justify and prioritize these enhancements. Similar to transmission additions, attractive return on investment is required. Restructuring, with formation of regional transmission organizations (transcos or ISOs), results in new control centers. This provides not only challenges, but “greenfield” opportunities for sophisticated wide-area control of large geographic coverage. Availability of fiber optic communication networks facilitate wide-area control. Meeting the challenges of power system operation under higher stress requires fidelity in steady-state and dynamic simulation. Despite significant efforts, large discrepancies still exist between simulation and the real world. Reference 2 describes problems, but four years later we are encountering similar problems, especially in simulation of oscillation damping. There is justified concern that simulation fidelity is poor compared to reliability criteria safety margins. Present Practice and State-of-the-Art in Stability Controls Power system stability and voltage controls are mainly local feedback control at generating plants, such as automatic voltage regulators including power system stabilizers and prime mover controls. Other feedback controls exist at special substation power electronic devices such as static var compensators. These controls are usually continuous (smooth). The controls are now highly developed, and new controls are implemented digitally [3]. Nevertheless, technological progress continues, with reference 4 being one example. Power system dynamic performance could be enhanced with replacement of legacy equipment such as PSS with noisy speed or frequency transducers and low gains. Power electronic devices, where justified, also offer powerful control performance (e.g., ref. 5). Further R&D is required for wide-area coordination of generator controls and transmission-level power electronic device controls [26]. Another class of controls, for both preventive and corrective actions, are mechanically-switched devices such as series and shunt capacitor banks. These are discrete (discontinuous controls), and the switching frequency is restricted. Circuit breaker operating time (two cycle opening, five cycle closing) are fast enough, for single operation, and can be fast enough for several bang-bang operations for control of low frequency oscillations (2–4 second period). Generator or load tripping are even more powerful discontinuous Bonneville Power Administration TOP/Ditt2, PO Box 491, Vancouver, WA 98666, USA, [email protected]. control for stability enhancement. At present, these discrete devices are mostly switched manually via SCADA, by simple local relays, or by event-driven wide-area controls (special protection systems/remedial action schemes). The devices and controls are widely used, and are usually cost-effective compared to power electronic devices. Shunt capacitor banks are low cost and have virtually zero losses. Modern all-film fuseless capacitor banks increase costeffectiveness [6,7]. New techniques for multiple-step banks are developed [8,9]. For perspective, Bonneville Power Administration has fourteen 500-kV capacitor banks (up to 460 MVAr at 550-kV), fifty-three 230-kV banks (up to 168 MVAr at 241-kV), and numerous 115-kV banks. BPA also has two large static var compensators and one thyristor controlled series capacitor, but this equipment is not cost-justified [10]. BPA and other power companies have very extensive widearea, event-driven stability controls for generator and load tripping, and for shunt and series reactive power compensation switching. PLCs are used for logic decisions, and transfer trip over microwave radio is used for communications. Fault-tolerant control center computers perform central logic. Redundancy ensures no single component will cause scheme failure. Events detected are usually outages of 500-kV lines. The event-driven wide-area stability controls, while very fast for transient stability and very reliable, have shortcomings. They obviously only operate for certain pre-determined disturbances within a portion of the transmission network. They are also expensive to install and modify. The complexity is such that BPA has a 24/7 “RAS Dispatcher.” For slower switching of shunt compensation (preventive control for voltage stability) manual switching has disadvantages, and automation is desirable. System operation is becoming ever more complex, with rapid growth of transac-